Now, the howto:translate and the forum discussion a few months ago seem to suggest that, since there's no direct usage of onee-sama here, I should be sticking to the original language 'eonni(언니)' for Flandre's "Let's go out and play!" i.e. "Eonni, let's go out and play!"

Since that makes little sense, and it makes even less sense to use Japanese honorifics when translating non-Japanese Korean dialog to English, I'm going to use Remilia's name for all eonni.

Alternatively, translate it as "big sister", given that that's what it means.

Maybe "Big sis" is a bit more appropriate, since it feels more familiar.

Deelles said:
Maybe "Big sis" is a bit more appropriate, since it feels more familiar.

But Remilia also says unni to Flandre, so I think he made the right choice. On another note, it pains me that while I can read Korean, I can't understand most of it (I definitely knew this one though). Maybe I should improve my vocabulary :I.

I believe when Remilia says "unni", she's using it to refer to herself. Translated without ordering, it would be more like, "big sister, isn't she drinking tea?", so the way Lich King translated it is correct.

Korean does involve a lot of third-person speeches, especially when a superior person speaks to a lower person.